1. Field of the Invention
This invention relates generally to fuel supply systems for engines, and more particularly to a method and apparatus for introducing fuel to an internal combustion engine both upstream and downstream of the intake manifold.
2. Description of the Related Art
While consumers continue to demand improved engine performance in terms of power, efficiency, reliability, and driveability, regulating bodies continue to impose stricter standards requiring further reductions in harmful exhaust emissions. As a result, engine development engineers must continually strive to develop new technologies that improve combustion performance while reducing exhaust emissions in order to meet market demands.
Gaseous fuels are becoming increasingly popular for use as engine fuels due to their inherently cleaner combustion, which provides the potential for lower regulated exhaust emissions. However, producing a reliable, efficient, cost-effective, heavy-duty, gaseous-fueled engine has proved to be a difficult task. Gaseous-fueled engines are typically spark-ignited, turbocharged, lean-burn engines derived from their dieseli counterparts, and the performance of such gaseous-fueled engines must mimic that of their diesel counterparts to ensure successful use in the intended applications. These lean-burn engines operate with lean fuel-air mixtures to achieve the optimal thermal efficiency (approaching that of a diesel engine) and to meet strict emissions standards (lower than that of a diesel engine). Such lean-burn engines typically operate very close to the lean flammability limit of the fuel-air mixture. For the purposes of this disclosure, the terms xe2x80x9clean flammability limitxe2x80x9d and xe2x80x9clean misfire limitxe2x80x9d are used interchangeably to denote the condition in which the fuel-air mixture contains just that amount of fuel which is necessary to sustain combustion and thereby prevent misfire.
To help quantify the lean flammability limit of a fuel-air mixture, a non-dimensional term known as xe2x80x9cequivalence ratioxe2x80x9d (xcfx86) is defined as the ratio of the actual fuel-air mass ratio to the stoichiometric fuel-air mass ratio. For example, a fuel-air mixture with an equivalence ratio of xcfx86=0.5 has 50% of the fuel of a stoichiometric mixture, regardless of the fuel type or composition. As noted above, lean-burn engines typically need to operate as close as possible to the lean flammability limit in order to achieve the desired engine performance and emissions levels. For instance, if the lean flammability limit of the fuel-air mixture is xcfx86=0.62 in a particular engine, the engine may need to operate at about xcfx86=0.65 to achieve the desired performance and emissions. If the engine operates slightly rich of the desired equivalence ratio, emissions will increase significantly and may result in non-compliance with applicable emissions regulations. On the other hand, if the engine operates slightly lean of the desired equivalence ratio, the engine may misfire, which adversely affects engine performance, driveability, and emissions. Thus, in a given engine, the equivalence ratio of the fuel-air mixture must fall within a certain range or xe2x80x9cwindowxe2x80x9d to provide acceptable engine operation. This equivalence ratio xe2x80x9cwindowxe2x80x9d can be very small and is dependent on many factors, as discussed below.
In a perfect world, all cylinders in an engine would have the same lean flammability limit and reach that limit at precisely the same time in operation. In the real world, however, this almost never occurs; instead, one cylinder will misfire before the others. Often, this one cylinder defines the lean operating limit for the whole engine. Likewise, on the rich side, one cylinder will almost always be richer than the others and produce higher emission levels than the others, thereby defining the rich operating limit for the whole engine. Therefore, the range of equivalence ratio that yields acceptable engine operation is smaller than if all cylinders reached their lean and rich limits at the same time.
Engine performance will improve, and the engine will produce lower emissions, if each cylinder operates at its optimum fuel-air mixture. This optimum mixture may be the same for each cylinder or it may vary among the cylinders, depending on the severity of the variation in the factors that affect combustion performance. Factors that cause variations in lean misfire limits among engine cylinders include: (1) fuel mal-distribution (fuel-air mixing on a macro scale); (2) fuel-air homogeneity (fuel-air mixing on a micro scale); (3) compression ratio; (4) volumetric efficiency (air mass in the cylinder); (5) cylinder wall temperature; (6) inlet mixture temperature; (7) coolant temperature; (8) residual exhaust fraction (left over combustion gas trapped in the cylinder); (9) in-cylinder air motion (swirl and tumble); and (lo) spark plug location and orientation. Those factors, along with basic fuel properties, define the lean flammability limit for each cylinder. The effects of the above factors may be minimized with careful design and development practices, but they cannot be eliminated.
Good homogeneity (mixing) is important for lean-burn combustion. Premix fuel systems, such as throttle-body injection (TBI) or carburetion systems, offer good fuel-air mixing, both on a macro and micro level. Good macro mixing provides good fuel mixture distribution to each cylinder. Good micro mixing provides good homogeneity of the mixture within each cylinder.
The mixing of fuel and air is a time-dependent phenomenon; the longer the mixing time, the more complete the mixing process. Premixing upstream of the intake manifold provides additional time compared to port injection systems. A premix fuel system offers the best control for factors (i) and (2) above, but it offers no control over factors (3) thru (10), which are cylinder-specific factors that require control of the fuel to each individual cylinder.
Multipoint fuel injection (MPI) offers control of fuel on an individual cylinder basis, which satisfies the control requirements for factors (3) through (10) above, but MPI has the disadvantages of higher fuel mal-distribution among cylinders due to injector flow variations, high sensitivity to those mal-distribution errors, reduced mixing time, and high fuel supply pressure. Historically, however, MPI systems have been used to control fuel on a macro level only (replacing the TBI); no successful attempt has been made to trim (tailor) the fuel flow on an individual cylinder basis. Thus, a need exists for a technique that will provide relevant feedback information from each cylinder to allow adjustment of the fuel injector for each respective cylinder.
Currently, no existing lean-burn fuel system provides the level of control required to simultaneously optimize engine performance, driveability, and emissions while maintaining a robust calibration. The existing engine calibration must, therefore, involve a compromise of the relevant parameters. The engine is usually calibrated richer than optimum to provide an acceptable margin from misfire and good driveability, but such a calibration results in higher than optimum NOx emissions. Current emissions regulations allow this type of compromise to exist, but lower emissions standards are in the process of being implemented, and soon such a rich calibration will not be an accepted practice.
In light of the foregoing problems, a method is needed whereby the effects of the aforementioned misfire factors can be controlled to the degree that they no longer impose unnecessary limitations on engine performance due to variations in the lean flammability limit among cylinders.
The fuel system of the present invention provides improved control of fuel delivery to an engine, preferably a gaseous-fueled engine, to enhance engine performance and driveability and simultaneously reduce exhaust emissions. The system disclosed herein introduces fuel, preferably gaseous fuel, to the engine at two locations: (1) upstream of the intake manifold (e.g., throttle body injection or carburetion) to provide premixing of a majority of the fuel with the air, and (2) near each intake valve (port injection) for trimming or tailoring the fuel flow to each cylinder based on the specific needs of the cylinder. Several calibration and control methods are described to maximize performance of the fuel system. Preferably, a misfire detection technique is used to determine the lean misfire limit of each cylinder, and a feedback control system allows the port fuel injectors to provide the appropriate xe2x80x9ctrimxe2x80x9d amount of fuel to each cylinder such that each cylinder operates at a predetermined margin from its lean misfire limit. The present invention thereby optimizes engine performance and maximizes the range of equivalence ratio in which the engine may operate and still comply with strict emissions standards.
It is an object of the present invention to provide an improved fuel control system for internal combustion engines whereby the fuel-air mixture for each cylinder of the engine may be maintained at an optimum equivalence ratio.
It is a further object of the present invention to provide a fuel control system for internal combustion engines whereby the fuel-air mixture for each cylinder of the engine may be maintained at a specified margin from the lean misfire limit of each respective cylinder.
It is still another object of this invention to provide a fuel control system for internal combustion engines that maximizes engine performance while enabling conformance with strict emissions regulations.
It is yet another object of this invention to provide a gaseous fuel control system for internal combustion engines that may utilize conventional liquid fuel injectors for port injection of gaseous fuels due to the reduced quantity of fuel injected in the port.
It is yet another object of this invention to provide a fuel control system having reduced fueling errors compared to those of conventional multi-point injection systems.
It is still another object of this invention to provide a fuel control system with improved injector durability due to lower fuel flow rates and lower fuel supply pressures.
It is still another object of the present invention to provide a fuel control system that benefits from the better fuel-air mixing associated with throttle-body injection systems due to the increased mixture transport time and that also benefits from the cylinder-specific accuracy associated with port injectors.
Another object of the present invention is to provide a fuel control system with reduced closed-loop cycle times as compared to conventional premix fuel systems.
Still another object of this invention is to provide a fuel control system with reduced starting times as compared to conventional premix fuel systems.
Yet another object of this invention is to provide a fuel control system that is compatible with liquid fuel storage systems with low fuel supply pressures.